1. Field of the Invention
The present invention relates to a measurement method using a solar simulator for high-speed, high-accuracy measurement of current-voltage characteristics (hereinafter also referred to simply as characteristics) of photovoltaic devices and the like and of photovoltaic devices panels.
2. Background of the Invention
The photoelectric conversion characteristics of photovoltaic devices, photo sensors and the like are measured by measuring the current-voltage characteristics of the photovoltaic devices under irradiance. In the measurement of the characteristics of photovoltaic devices, a graph is set up with voltage on the horizontal axis and current on the vertical axis and the acquired data is plotted to obtain a current-voltage characteristics curve. This curve is generally called an I-V curve.
As the measurement methods, there are methods that use sunlight as the irradiating light and methods that use an artificial light source as the irradiating light. Of the methods that use an artificial light source, methods that use a fixed light and methods that use a flash are described in, for example, Japanese Patent No. 2886215 and Laid-open Japanese Patent Application No. 2003-31825.
Conventionally, with the commercialization of photovoltaic devices, and particularly with photovoltaic devices with large surface areas, the current-voltage characteristics are measured under a radiation irradiance of 1000 W/m2, which is sunlight standard irradiance. Measured values are corrected mathematically by formula so as to compensate for when irradiation during measurement exceeds or falls short of 1000 W/m2.
In addition, measurement of the current-voltage characteristics of large-surface-area photovoltaic devices require irradiation of light with an irradiance of 1000 W/m2 to a large-surface-area test plane uniformly. As a result, when using an artificial light source, for example, a high-power discharge lamp capable of providing several tens of kilowatts per square meter of radiation surface is required. However, in order to make such a high-power discharge lamp provide a fixed light it must be provided with a steady supply of high power. As a result, a very large scale equipment is required, which is impractical.
In addition, with a solar simulator that uses a steady light, a xenon lamp, a metal halide lamp or the like for continuous lighting is used for the light source lamp. FIG. 4 is a diagram showing the relation between irradiance and time for such lamps. As shown in this drawing, it usually takes several tens of minutes or more from the start of lighting of such lamps until irradiance stabilizes. Moreover, unless lighting is continued under the same conditions the irradiance does not reach saturation, and therefore a great deal of time is required until measurement is started. On the other hand, as the accumulated lighting time grows by long hours of lighting, the irradiance tends to decrease gradually, and thus the irradiance characteristics are not stable. In addition, the radiation of the light on the photovoltaic devices under measurement is conducted by changing shielding and irradiation of light with the opening and the closing of the shutter. Thus, the irradiation time required for the devices under test depends on the operating speed of the shutter, and often exceeds several hundred milliseconds. As the irradiation time lengthens the temperature of the photovoltaic devices itself rises, thus making accurate measurement difficult.
With a solar simulator that uses a fixed light, although it is necessary to maintain continuous lighting in order to stabilize the irradiance, doing so causes the temperature inside the housing that contains the light source to increase sharply. In addition, the parts inside the housing are constantly exposed to light, which causes the optical components (mirrors, optical filters, etc.) to deteriorate.
Further, once a fixed light source lamp is turned off and turned on again, it takes several tens of minutes for the irradiance to reach saturation. In order to avoid this, usually the fixed light source lamp is kept on and used as is. As a result, however, the accumulated lighting time of such fixed light lamps adds up easily, resulting in a tendency for such lamps to reach the end of their useful lives relatively quickly.
Therefore, when using a fixed light-type solar simulator in a photovoltaic devices module production line, the number of lamps that burn out is added to the running cost, which increases not only the cost of measurement but also the cost of production.
In addition, with a fixed light solar simulator, the length of time during which light from the light source irradiates the photovoltaic devices under measurement is relatively long. As a result, when I-V curve measurements are repeated for the same photovoltaic devices, the temperature of that photovoltaic devices rises. As the temperature of the photovoltaic devices rises its output voltage tends to decrease, and it is known that a rise in temperature also decreases maximum output Pmax.
In general, measurement of the photovoltaic devices current-voltage characteristics requires indicating standard test condition values. Here, the temperature of the photovoltaic devices in under standard test conditions is 25° C. and the radiation irradiance is 1000 W/m2. The measurement of the current-voltage characteristics of photovoltaic devices by a solar simulator is carried out with the temperature range of the photovoltaic devices in the range of 15° C.-35° C. The temperature is corrected to the 25° C. that is the reference temperature using the measured temperature of the photovoltaic devices. The correction formula used for this purpose is prescribed by industry standard.
However, measurement of the temperature of the photovoltaic devices has the following problems and is not a simple matter. Photovoltaic devices used to supply power to general houses have a laminated construction, with a glass surface beneath which are EVA (ethyl vinyl acetate), the photovoltaic devices cell, and more EVA, with a plastic backing sheet on the back of the photovoltaic devices. When the temperature of photovoltaic devices with this sort of laminated structure is measured on the production line, only the temperature at the surface of the back sheet or at the front glass surface is measured. Therefore, even when the photovoltaic devices cell accepts the light irradiated from the solar simulator and the temperature temporarily rises, it is extremely difficult to measure the temperature of the photovoltaic devices cell itself correctly. This situation makes it difficult to measure the temperature of the photovoltaic devices cell accurately, and also makes it difficult to correct for the temperature correctly.
Accordingly, a method that measures the current-voltage characteristics of large-surface-area photovoltaic devices using a flash instead of a fixed light has been proposed. More specifically, there are two measurement methods: one is a single-flash light measurement method that uses a single flash light with relatively long flash duration and another one is a short-pulse flash measurement method that uses multiple flashes with a short flash duration. In both cases, a xenon lamp is used as a pseudo sunlight light source that generates the flash.
With either flash-based measurement method for measuring the current-voltage characteristics of photovoltaic devices, the problem of a rise in the temperature of the photovoltaic devices during measurement that arises in fixed light measurement methods is substantially nonexistent, and therefore errors caused by a temperature rise in the temperature of the photovoltaic devices cell do not arise easily.
In addition, in a solar simulator that performs data acquisition using a flash the flash duration is reduced, which has the advantage of alleviating the deterioration of the optical components of the fixed light solar simulator described above, which makes the lamp life relatively long.
FIG. 5 is a diagram showing a single flash waveform. Single flash is a method of causing a xenon lamp to fire using a direct current power source capable of outputting a large current. At the beginning of the light pulse waveform there is a portion in which the irradiance fluctuates sharply, after which the irradiance is constant. In a measurement method that uses a single flash, during the time when the irradiance becomes constant in the pulse waveform, the output of the photovoltaic devices are measured by acquiring data on current and voltage output from the photovoltaic devices under measurement while controlling a load of the photovoltaic devices.
However, it is known that the irradiance of a xenon lamp flash is uneven, and the flash must have an irradiance that is within a permissible range of ±5 percent. The irradiance is then corrected for according to the irradiance during flash, but when the characteristics of the photovoltaic devices are unknown and the permissible range is large, measurement accuracy deteriorates.
In addition, in order to obtain I-V characteristics curves with a single flash by sweeping the load of the photovoltaic devices, it is necessary to create a long pulse exceeding 100 milliseconds. In order to generate such a long pulse flash, the rest interval between one flash and the next must be long as well. As a result, if in the first generation of the flash the irradiance is insufficient and cannot be completely corrected, a long wait must be endured until the next generation of the flash. In addition, in order to fire a long pulse the load of the lamp must be large, which in turn shortens the life of the lamp.
In a measurement method that uses multiple short pulse flashes, the load of the light source lamp needed to fire the flash is small, and thus the flash can be fired at short intervals. In addition, because the flash duration is short, there is little change in conditions inside the lamp (for example, temperature), and therefore peak irradiance stabilizes easily. The photovoltaic devices under measurement are subjected to a light pulse that is short, and therefore its temperature does not rise much, either.
However, this type of short pulse flash measurement has the following problems. FIG. 6 is a diagram showing a waveform of a short pulse flash. As shown in this drawing, the waveforms of multiple flashes are mountain shaped with no flat part at their peaks (with the duration at the base of the mountain-shaped waveform being approximately 1 millisecond). As a result, in a single generation of a flash, only a single set of data (consisting of irradiance on the one hand and photovoltaic devices output current and voltage on the other) can be acquired. Further, when measuring a slow-response photovoltaic devices, the output response of the photovoltaic devices cannot track the irradiance waveform, which means that sometimes the measured output is lower than the true output.